There has been considerable interest in recent years in the use of biodegradable polymers to address concerns over plastic waste accumulation. The potential worldwide market for biodegradable polymers is enormous (>10 B lbs/yr). Some of the markets and applications most amenable to the use of such biopolymers involve those having single, short use applications, including packaging, personal hygiene, garbage bags, and others. These applications, although poorly suited for recycling, are ideally suited for biodegradation through composting.
PHA biopolymers are polyesters produced by numerous microorganisms in response to nutrient limitation. The commercial potential for PHA spans many industries, and is derived primarily from certain advantageous properties which distinguish PHA polymers from petrochemical-derived polymers and other polymers derived from renewable resources, namely excellent biodegradability and/or natural renewability compared to the petrochemical-derived polymers, and hydrophobicity compared to other polymers derived from renewable resources.
Widespread use and acceptance of PHA, however, has been somewhat hindered by certain undesirable chemical and physical properties of these polymers. For example, PHA is one of the most thermosensitive of all commercially available polymers. As such, a dramatic rate of polymer degradation is observed at the temperatures typically required for conventional processing of PHA into end-products such as films, coatings, fibers etc. An additional limitation of the potential utility of PHA polymers relates to the observation that certain characteristics such as ductility, flexibility and elongation of the polymer diminish over time. This rapid “aging” of certain PHA-derived products is unacceptable for many applications in that the products fail to maintain adequate structural integrity for their intended useful life. Thus, the success of PHA as a viable alternative to both petrochemical-derived polymers and to non-PHA biodegradable polymers, will depend upon novel approaches to overcome the unique difficulties associated with PHA polymer processing and with products derived therefrom.
One important application of biodegradable polymers is in the area of films. Films with properties suitable for diverse applications, however, have been difficult to prepare using PHA polymers due to their frequently unacceptable mechanical characteristics, for example poor melt strength, rapid aging, and brittleness.
GB 2,243,327 teaches that biodegradable PHA polymers, such as polyhydroxybutyrate-co-valerate (PHBV) copolymer, cannot be formed into thin films while maintaining the required strength and tear resistance for applications such as a diaper backsheet. This problem was addressed by combining a thin biodegradable thermoplastic polymer film into a sheet laminate with at least one layer of a fiber substrate comprised of biodegradable fibers. However, a cast film of PHBV (6.5% HV) 20–24 microns thick when laminated with a rayon nonwoven showed elongation at break of less than 10% in the machine direction which was less than that for the rayon portion alone.
U.S. Pat. No. 4,880,592 discloses a means of achieving a free-standing PHBV film for diaper backsheet applications by coextruding the PHBV between two layers of sacrificial polymer, for example polyolefins, stretching and orienting the multilayer film, and then stripping away the polyolefin layers after the PHBV has had time to crystallize. The PHBV film is then laminated to either water soluble films or water insoluble films such as PVDC or other polyolefins.
WO 94/00293 discloses multilayer films wherein the PHBV component may be coextruded as an internal layer surrounded by outer layers of films (e.g. biodegradable films) to be used in the application (e.g. diaper films) so that the external layers are not stripped away from the PHBV layer. They remain as an integral part of a biodegradable multilayer film.
EP 736,563 describes a process for producing cast and blown films from PHBV copolymers that have been compounded with plasticizers, nucleating agents and/or other additives. No specifications of the copolymer were disclosed other than an HV content of 4–16%. The molecular weights of the PHBV powder used in the examples ranged up to 520,000 before compounding and extrusion into pellets. In their disclosed process, films of the compounded PHBV were extruded on a preformed supportive bubble of EVA-resin or LDPE with MFI less than 2.5 g/10 min.
Thus, attempts to use PHA in the production of films have generally resulted in unacceptable elongational properties for use in a continuous process and/or a requirement for the presence of one or more non-PHA polymer layers in order to improve film processibility and/or properties. It is desired, therefore, to have a method to produce PHA films by a continuous process that have improved overall mechanical characteristics as required for the numerous applications in which films are typically employed. Furthermore, it is desired that the means to produce improved PHA films be a simple and rapid process that is easily amenable to large scale production, where the PHA films do not require the presence of non-PHA polymeric materials to achieve acceptable processing performance and/or product properties.